Certain drugs whose molecular structure contains several potential sites for oxidation are selectively hydroxylated by the seemingly non-specific drug-metabolizing systems of mammalian liver microsomes which act upon a broad spectrum of different substrates. It has been generally accepted that binding of the different substrates to ferricytochrome P450 is a required step in their oxidative catabolism. However, it is not clear how these agents are bound, whether the binding is altered by other components of the electron transfer system, nor whether selective oxidation is related to the mode in which the drug is bound. Recently, we found that hemoglobin could substitute equieffectively for P450 in certain drug hydroxylation reactions, and that the substrate-ferrihemoglobin complex was not important catalytically; rather, this reactivity might be correlated with direct binding to the oxyferrohemoprotein and consequent changes in its properties. NMR and UV difference spectroscopy, measurements of oxygen affinity and specific assays for products will be used to study the interactions of a variety of chemical substances with several hemoproteins and reconstituted hemoprotein and reductase systems with the overall goal of understanding the bases for the subtle differences in the physiological functions of hemoproteins and how they may relate to drug metabolism or toxicity.